The present invention is particularly suitable for power steering systems of vehicles, such as tractors or the like, and it is therefore described in that context. In this regard, the concern and problem relate to attempts to effect a smooth and linear change in the flow of fluid in the system and to thereby avoid jerky steering response. In this respect, the invention of the present hydraulic system is arranged to sense the load on the system and to automatically respond to the load which is actually variable or changing, and the final result is to have the entire system smooth in its operation and completely capable of producing the necessary force to overcome the resistance and perform the work desired.
In achieving the aforementioned, and in providing the hydraulic system of this invention, the present system is arranged so that it senses and responds to the load pressure during actuation, such as during the steering of a vehicle, and it also senses and responds to the return hydraulic pressure when the system is in a neutral condition. The result is that the operating or main valve of the system always has a constant pressure drop across it, and thus smooth and precise hydraulic operation is achieved. Accordingly, the main or metering valve will have better metering characteristics, that is, it will have smoother and more linear change in flow for each change in the valve position, and this is true since the pressure across the valve, that is from inlet to the outlet, can be and is a constant value. In achieving these objectives, the main or metering valve has a pilot port which is internally connected to the valve outlet leading to the load-responsive actuator, and thus the hydraulic pressure at the pilot port is actually the load pressure. That pressure is communicated to a compensator which is hydraulically connected to a fixed displacement hydraulic pump, and the compensator responds to the pilot port hydraulic pressure to thereby control the pump outlet pressure and accordingly effect the constant pressure drop across the metering valve, as mentioned above.
Still further, the present invention provides a hydraulic load-sensing system which is particularly useful for hydraulic steering apparatus in a vehicle or the like, and with the system being reliable and sensitive and precise, and with the system requiring only a minimum of elements and without any of the elements being complex and unreliable.
Still further, the present invention provides a load-sensing hydraulic system wherein the power to operate the system is only a minimum, such as only the power actually required, and the hydraulic fluid is therefore not unduly heated, and the prime mover is not operating beyond the requirements for the actual production of constructive effort by the system. Also, where the present system is used for a power steering system of a vehicle, the load pressure changes, such as might be caused by the vehicle encountering an obstacle while steering, will be sensed in the system and the system will respond to the pressure changes to effect efficient action and to accomplish the aforementioned desirable results. Also, where the pressure drop across the metering valve is a constant, accomplished as disclosed herein and as mentioned above, the rate at which the steering cylinders of a hydraulic system employed in power steering of a vehicle, is a function only of the rate at which the steering wheel itself is turned, and the load on the system will have no effect on the steering, at least up to the limit of a pressure relief valve which may be employed in the system.
Still further, the present invention provides a hydraulic system of a load-sensing type which may be adjusted to thereby control the time required for the system to respond to changes in the load on the system. Also, in the hydraulic system of this invention, all of the output of the hydraulic pump is bypassed, except for the exact amount of hydraulic fluid required by the rate at which the metering or steering valve itself is turned.
Other objects and advantages will become apparent upon reading the following description in light of the accompanying drawing.